Fertilization ability of mammalian sperm is influenced by factors such as morphology, biochemical constitution and motility. Fertile mammalian sperm, however, are difficult to unambiguously identify so only indirect evidence exists for the relationship of the above factors to fertility. The proposed research aims to elucidate direct causes of sterility by investigating spermatogenesis and sperm biochemistry and motility in a fruitfly, Drosophila pseudoobscura. Males of this species predictably and simultaneously produce easily identifiable fertile motile sperm and sterile sperm that may be motile or nonmotile. Specific aims of this research are to describe spermatogenesis to determine if (and when) development is altered between fertile and sterile sperm, and to examine and characterize variation in biochemical constituents, such as proteins and carbohydrates, between fertile and sterile sperm. Both proper sperm development and biochemistry may affect motility. I will use standard cytological, molecular, and microscopic methods described in mammalian and Drosophila research to accomplish these aims. The proposed study is applicable to human reproductive biology. Approximately 40% of infertility is due to male sterility. Elucidating factors causing sterility may indicate ways that sterility could be alleviated or promoted for contraception.